1. Field of the Invention
The present invention relates to transformer cores and especially to three-phase cores comprising three frames of rings of transformer plate defining yokes in, for example, horizontal triangles and vertical legs extending between corners of the yokes. The invention also relates to single-phase shell cores having many rings, frame cores having two frames and two yokes, inductors, and components for the foregoing and transformers.
2. Description of the Prior Art
Transformer cores are almost solely made of transformer plates laid edge to edge to an EI form or ring. Some of them are made of cut rings and called C-cores. Others are wound to two rings inside a larger ring and cut to two E-core parts used for three-phase transformers.
Toroid transformers have a ring core which is not cut and is the only practical exception to the EI form. Small shell single-phase transformers and large three-phase transformers use the EI form of core.
A common three-phase transformer core will now be described. Virtually all have six coils, which by means of wires are wound on a cylinder forming three rod-shaped windings. The cores are composed of a multitude of thin, rectangular plates of electromagnetic material, which are stacked side-by-side with their long sides in alignment to form each of the legs.
The E-shaped plates form one yoke, and three short legs each extending its body into one of the transformer coils. Each leg faces a leg from a yoke at the opposite end of the coil. Thus, there is a core leg extending through each of the three sets of wound coils encircling the legs. The coils are bridged by one yoke at each side of the coils.
The plates of the core are thin sheets of metal, assembled in place, one sheet at a time, until an entire core is put together. This is a slow, labor intensive process.
The EI cores are inefficient to operate, and electrical losses occur at the juncture of many mating edges between the plates. That is, EI stacked cores in general have the drawback that the magnetic field has to cross small gaps between the edges from plate to plate.
There are further losses in the four outer corners of a complete three-phase transformer where field radiation occurs, since there is no ready path for the magnetic fields to flow. Further, the yokes are made of core material that is not encircled by coils and therefore does not contribute to the efficiency of the transformer, but to the contrary the consequence is that material and labor to form the yokes can be regarded to be wasted.
When the operation of a standard three-phase transformer is commenced, there are very high current losses. There are thus high losses during start-up as well as under load for standard three-phase transformers due to the conventional three-phase transformer cores.
The EI cores used in three-phase transformers have numerous other shortcomings. They vibrate and hum during operation. They set up electromagnetic radiation that is easily detectable at about five feet from typical three phase transformers. Due to e.g. electromagnetic forces in the space between the edges of the plates, there [is] will be noise in the core. Conventional three-phase transformers generate excessive amounts of heat, and means must be employed to cool them, requiring an excessive amount of cooling fluid.
As noted above, the inefficiencies of three-phase transformers requires each of them to have a large size. This requires them to be larger in both width and height. Large transformers are difficult and expensive to transport, due both to their size and height. In addition, shipping large transformers sometimes results in damage due to their instability. For example, large transformers have at times been unable to be shipped to offshore installations, and such shipping if possible is expensive. Designers of transformer cores have striven to obtain legs with an essentially circular cross-section because that gives the best efficiency of the final transformer. That is, transformer windings are nearly always cylindrical, having an interior void within the windings with a circular cross section. The core designer wants to fill that void. This is true for both three-phase and single-phase transformers. However, there is always a trade-off between efficiency and production requirements, leading to non-optimal transformer cores with non-circular legs.
U.S. Pat. No. 4,557,039 (Manderson) discloses a method of manufacturing transformer cores using electrical steel strips having approximately a linear taper. By selecting a suitable taper, a hexagonal or higher order approximation of a circular cross section for the legs of the cores is produced. However, the tapered strips are extremely difficult and time-consuming to produce, and the design is particularly not well adapted to large-scale commercial production.
In FIGS. 1, 1a-b is shown a prior art three-phase transformer core according to Manderson, generally designated 10. The core has a general delta-shape, as is seen in the isometric view of FIG. 1, with three legs interconnected by yoke parts. In FIG. 1a, a cross-sectional view of the core is shown before final assembly. The core comprises three identical ring-shaped parts 12, 13 and 14, the general shape of which appears in FIG. 1 Each ring-shaped part fills up one half of two legs with hexagonal cross-sections, see FIG. 1a, thus totaling the three legs of a three-phase transformer. The ring-shaped parts are initially wound from constant width strips to three identical rings 12a, 13a, 14a with rhombic cross-sections comprising two angles of 60xc2x0 and two angles of 120xc2x0. These rings 12a-14a constitute the basic rings. The orientation of the strips also appears from FIGS. 1a and 1b. 
Outside of the basic ring in each ring-shaped part there is an outer ring 12b, 13b, 14b of a regular triangular cross-section. The outer rings are wound from strips with constantly decreasing width. When the three ring-shaped parts 12-14 are put together, see FIG. 1b, they form three hexagonal legs on which the transformer windings are wound.
A drawback with this solution is that every size of transformer requires its own cutting of the strips. Also, the outer rings 12b-14b are made of strips with decreasing width, leading to waste and also making the transformer according to Manderson very difficult to manufacture. Another drawback is that the design is not self-supporting, i.e. the ring-shaped parts tend to move in respect of each other.
U.S. Pat. No. 2,544,871 (Wiegand) discloses a transformer core with three legs, each leg being made from two ring-shaped parts and an auxiliary ring shaped part. There are thus nine ring shaped parts, three of which are used in an inefficient way, making Wiegand an expensive and impractical device.
Transformer cores are also described in the following documents: Swedish Patent No. 163797, U.S. Pat. No. 2,458,112, U.S. Pat. No. 2,498,747 and U.S. Pat. No. 2,400,184. However, the above mentioned problems are not overcome by the cores described in these documents.
The difficulty and expense of making transformer cores having a ring with a decreasing width has rendered this proposed transformer core construction totally impractical, and no such cores are known to exist in commercial use. Despite the recognition that transformer cores should nearly fill the circular interior of transformer windings, none has hitherto been proposed which is practical and economical. The design of such a transformer core would be of tremendous importance with respect to both three-phase and single-phase transformers. In addition, the provision of similarly designed inductors would be a most significant contribution to the art.
The electromagnetic and mechanical shortcomings of present three and a single-phase transformer core is very significant considering the number of transformers sold and in use throughout the world. The following figures demonstrate this. They were taken from the xe2x80x9cThe World Market for Transformers 2000xe2x80x9d published by Golden Reports, of 109 Uxbridge Road, Ealing, London W5 5TL, United Kingdom. In 1999, the market in the United States for transformers was 2,664 units. The total for the rest of the world was about 4,828 transformers. The world market for transformers from 1994 through 1999, was as follows:
The World Market for transformers is growing, as shown in the following table:
with an average annual growth of 4.1%. The sales of transformers is extremely large, as the following demonstrates:
An object is to provide a transformer core, which is easy to manufacture and avoids material waste.
Another object of the present invention is to provide a transformer core wherein the energy losses are significantly reduced.
Another object of the invention is to provide a transformer core for a transformer winding which is electromagnetically efficient.
It is yet another object of the invention to provide a transformer core for a transformer winding which substantially fills the void in the winding.
A still further object is to provide a transformer core for substantially filling a transformer winding comprising an assembly of wound metal strips, each of the metal strips having constant widths.
It is an additional object to provide a ring shaped core for transformers made of a combination of rings of wound strips of ferromagnetic material, each component ring having an equal width.
It is still a further object of the invention to provide ring shaped transformer cores for three-phase transformers made of set of wound metal strips, the metal strip of each component of the set having an equal width.
Another object is to provide a ring-shaped transformer core having straight legs and yokes curved at their intersection with straight legs for rendering the flow of magnetic fields efficient, the legs having polygonal cross sections and being made of wound metal strips, each strip having a constant width.
It is yet another object of the invention to produce a three-phase transformer core with rings made from transformer plate of a constant width for the respective rings and with a controlled thickness, the core having legs whose cross-section is composed of rhombs and/or rhomboids, including squares and rectangles, forming a polygon.
Another object is to provide a three-phase transformer core made of rings of offset transformer plate, the core having legs composed in cross-section of rhombs and/or rhomboids, forming a regular polygon having at least six sides.
It is an object of the invention to provide the foregoing transformer cores that can be produced in commercial volumes having high quality as transformer cores and being produced in a practicable manner.
Another object is to provide a transformer core that does not suffer from the relatively high electromagnetic losses at junctures of adjoining sheet metal plates, end corners and angled corners of EI cores presently used in three-phase transformer cores.
Still yet another object is to reduce the noise from electromagnetic forces between the plates.
A further object is to provide a transformer core that does not hum or vibrate, as do transformer cores in current use.
Another object is providing a transformer core that does not suffer start up losses, as do conventional transformer cores.
A further object is to provided a practical and efficient transformer core for three-phase transformers which enables the windings to be arranged in a compact orientation, i.e. at the corners of an imaginary triangle.
It is yet still another object of the invention to provide a three-phase transformer core made from rings of offset transformer plate, the rings cooperating to define legs in a delta arrangement for extending through transformer windings in a delta arrangement.
It is an additional object of the invention to provide a three-phase transformer core which is more effective and efficient than prior transformer cores for three-phase transformers, yet does not require the manual labor to assemble the core as do conventional EI cores.
Still another object is to provide an improved single-phase transformer core having as a cross-section a polygon of at least six sides.
An additional object is to provide a single-phase transformer core having in cross-sections rhombs and/or rhomboids, defining a polygon having at least six sides.
Yet another object is to provide an improved inductor core having a polygonal cross section with wound rings, each ring being made of a metal strip with equal width.
A further object of the invention is to provide an improved inductor core having yokes of polygonal cross section made of offset laminates of transformer plate, each of equal width, and made from metal of constant width.
It is a general object to make inductors of excellent electromechanical efficiency that can be made economically using metal strips of constant non-varying widths.
It is another object of the present invention to provide an improved frame (as explained below) for transformer cores.
An additional object is to provide an improved frame for inductor cores.
A further object is to provide an improved transformer.
A still further object of the present invention is the provision of an improved inductor.
Another object is to provide a method of manufacturing a transformer that is well adapted for large-scale production.
Other objects will occur to those skilled in the art from the description to follow and from the appended claims.
The invention is based on the realization that a transformer core with one or more regularly multi-edged legs with more than four edges can be assembled from rings of strips with constant width.
An important aspect of the present invention with respect to three-phase transformer cores is to make or fill rhombic cross-sections in three legs in a triangular pattern using three frames, each frame being composed of at least two rings of wound offset transformer plate, where each frame forms part of two legs. Each leg has a polygonal cross-section made of rhombs and in some cases, rhomboids. The straight or leg portion of two frames cooperate to form a transformer core leg, the leg having the polygonal cross-section. The leg portion of the cooperating parts of two frames each define one half of the desired polygonal cross-section, and together they form the desired full polygonal cross-section. While the simplified polygonal cross-section is a hexagon, any number of even sides, six or above, can be prepared pursuant to the invention. Moreover, while the combined leg portions can cooperate to form the desired polygonal cross-section, with each of the two frames forming one half of the desired polygonal cross-section, it is also possible pursuant the invention to form the desired rhombic cross-section using frames to each contribute one third of one desired rhombic cross-section.
Another important aspect of some of the preferred forms of the invention is that the transformer core is made of frames of rings of offset or splayed material as defined below, each ring being made of wound sheets of transformer plate of a constant width. Each frame has surfaces which match corresponding surfaces of another frame, and they connect and lock the frames together. This differs from the prior art as shown in FIG. 1 where the surfaces are flat or even, because such surfaces can slip apart and make it difficult to keep the frames in a tight fixed position during subsequent manufacturing stages and also makes such transformer cores less stable and shock resistant during transport and installation. The traditional E-shaped core of stacked thin sheets is very vulnerable and is likely to loose some of its initial efficiency when moved around in the factory and during transport and installation. The edges of the prior art plates separate somewhat when a core or transformer is moved around, as is well known in the artxe2x80x94and the preferred embodiments do not suffer this disadvantage.
This differs from U.S. Pat. No. 2,544,871 to Wiegand where three cores and three auxiliary cores cooperate to form three transformer legs, since it takes two rings and an auxiliary ring to form one transformers leg. The three auxiliary cores are not necessary in the present invention.